Electromagnetic-wave-absorbing sheet

ABSTRACT

An electromagnetic wave absorbing sheet is provided that can adequately absorb electromagnetic waves at high frequencies in and above the millimeter wave band, can have excellent flexibility, and can easily be placed in any desired portion.The electromagnetic wave absorbing sheet includes an electromagnetic wave absorbing layer 1 containing a magnetic iron oxide 1a that magnetically resonates at frequencies in and above the millimeter wave band and a resin binder 1b. The electromagnetic wave absorbing sheet absorbs radiated electromagnetic waves by magnetic resonance of the magnetic iron oxide. The electromagnetic wave absorbing sheet has a flexibility evaluation value F (g/mm2) of more than 0 and 6 or less, which is determined by measuring an applied weight (g) that is required to bend a ribbon-like electromagnetic wave absorbing sheet in the elastic deformation region so that a distance d between the inner surfaces of the ribbon-like sheet at a position L spaced 10 mm from the bent portion of the ribbon-like sheet is 10 mm, and dividing the applied weight (g) by a cross-sectional area D (mm2) of the ribbon-like sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of co-pending applicationSer. No. 16/347,416, filed on May 3, 2019, which is the National Phaseunder 35 U.S.C. § 371 of International Application No.PCT/JP2017/039696, filed on Nov. 2, 2017, which claims the benefit under35 U.S.C. § 119(a) to Patent Application No. 2016-216290, filed in Japanon Nov. 4, 2016, all of which are hereby expressly incorporated byreference into the present application.

TECHNICAL FIELD

The present disclosure relates to an electromagnetic wave absorbingsheet for absorbing electromagnetic waves. In particular, the presentdisclosure relates to an electromagnetic wave absorbing sheet that hasexcellent flexibility, includes a magnetic material that absorbselectromagnetic waves by magnetic resonance, and absorbs electromagneticwaves at high frequencies in and above the millimeter wave band.

BACKGROUND ART

Electromagnetic wave absorbing sheets have been used to avoid theinfluence of leakage electromagnetic waves, which are emitted from,e.g., electric circuits to the outside, or undesirable reflectedelectromagnetic waves.

In recent years, centimeter waves with frequencies of several gigahertz(GHz) have been used in, e.g., mobile communications such as mobilephones, wireless LAN, and electronic toll collection (ETC) systems.Moreover, studies have made progress on the technology usingelectromagnetic waves not only in the millimeter wave band from GHz to300 GHz, but also in a higher frequency band than the millimeter waveband, corresponding to a frequency of 1 terahertz (THz).

According to the technical trend toward electromagnetic waves withhigher frequencies, electromagnetic wave absorbers for absorbingunwanted electromagnetic waves as well as the electromagnetic waveabsorbing sheets are increasingly required to be able to absorbelectromagnetic waves at frequencies between GHz and THz.

As an example of the electromagnetic wave absorber that absorbselectromagnetic waves at higher frequencies, Patent Document 1 proposesan electromagnetic wave absorber that has a packed structure ofparticles having an epsilon iron oxide (ε-Fe₂O₃) crystal as the magneticphase. This electromagnetic wave absorber exhibits electromagnetic waveabsorption performance in the range of 25 GHz to 100 GHz. PatentDocument 2 proposes a sheet-like oriented body. The sheet-like orientedbody is produced by mixing fine particles of epsilon iron oxide with abinder, and drying and curing the binder while an external magneticfield is applied, so that the magnetic field orientation of the epsiloniron oxide particles can be improved.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2008-60484 A

Patent Document 2: JP 2016-135737A

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

When predetermined electronic circuit components are shielded fromleakage electromagnetic waves from a source that emits electromagneticwaves, or protected from unexpected reflected waves or externallyincident electromagnetic waves, an electromagnetic wave absorbingmaterial should be provided to cover the circuit components to beprotected. In this case, it is often easier to attach theelectromagnetic wave absorbing material to the outer or inner surface ofthe existing member such as a case of electronic equipment that housesthe circuit components to be protected than to form the electromagneticwave absorbing material into a shape that covers the circuit componentsto be protected.

As described above, if the circuit components to be protected aresurrounded by the electromagnetic wave absorbing material using theexisting member, it is very important that the electromagnetic waveabsorbing material is closely arranged without any space. Moreover, ifthe electromagnetic wave absorbing material is placed on a curvedsurface, an electromagnetic wave absorbing sheet is more useful than asolid electromagnetic wave absorber because the former can easily followthe curved surface. When the electromagnetic wave absorbing sheet isclosely arranged, it may sometimes need to be rearranged so as not toleave any space. Thus, there is a possibility that the electromagneticwave absorbing sheet that has been sticking to a surface will beforcibly peeled off and can be very distorted. Therefore, theelectromagnetic wave absorbing sheet should be flexible enough towithstand such a rearrangement. However, the conventionalelectromagnetic wave absorbing material that can absorb electromagneticwaves at high frequencies in and above the millimeter wave band does nothave sufficient flexibility even if it is in the form of a sheet.

In view of the demand for the electromagnetic wave absorbing sheet, itis an object of the present disclosure to provide an electromagneticwave absorbing sheet that can adequately absorb electromagnetic waves athigh frequencies in and above the millimeter wave band, can haveexcellent flexibility, and can easily be placed in any desired portion.

Means for Solving Problem

To solve the above problem, an electromagnetic wave absorbing sheet ofthe present disclosure includes an electromagnetic wave absorbing layercontaining a magnetic iron oxide that magnetically resonates atfrequencies in and above a millimeter wave band and a resin binder. Theelectromagnetic wave absorbing sheet absorbs radiated electromagneticwaves by magnetic resonance of the magnetic iron oxide. Theelectromagnetic wave absorbing sheet has a flexibility evaluation valueF (g/mm²) of more than 0 and 6 or less, which is determined by measuringan applied weight (g) that is required to bend a ribbon-likeelectromagnetic wave absorbing sheet in an elastic deformation region sothat a distance d between inner surfaces of the ribbon-like sheet at aposition L spaced 10 mm from a bent portion of the ribbon-like sheet is10 mm, and dividing the applied weight (g) by a cross-sectional area D(mm²) of the ribbon-like sheet.

Effects of the Invention

The electromagnetic wave absorbing sheet of the present disclosureincludes a magnetic iron oxide that magnetically resonates atfrequencies in and above the millimeter wave band, and has a flexibilityevaluation value F of more than 0 and 6 or less. Thus, theelectromagnetic wave absorbing sheet is highly practicable and can havehigh flexibility and electromagnetic wave absorption properties toabsorb electromagnetic waves at high frequencies in and above themillimeter wave band.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a cross-sectional view illustrating the configuration of anelectromagnetic wave absorbing sheet of an embodiment.

FIG. 2 is a diagram for explaining the electromagnetic wave absorptionproperties of an epsilon iron oxide in which a part of the Fe site issubstituted.

FIG. 3 is a model diagram for explaining a measurement method of aflexibility characteristic value F based on the magnitude of a weightapplied to a sheet when the sheet is bent.

FIG. 4 is a diagram for explaining the relationship between the size ofan epsilon iron oxide powder contained in an electromagnetic waveabsorbing layer and the degree of bending of an electromagnetic waveabsorbing sheet.

FIG. 5 is a cross-sectional view illustrating the configuration of areflection-type electromagnetic wave absorbing sheet, which is amodified example of the electromagnetic wave absorbing sheet of thisembodiment.

DESCRIPTION OF THE INVENTION

An electromagnetic wave absorbing sheet of the present disclosureincludes an electromagnetic wave absorbing layer containing a magneticiron oxide that magnetically resonates at high frequencies in and abovethe millimeter wave band and a resin binder. The electromagnetic waveabsorbing sheet absorbs radiated electromagnetic waves by magneticresonance of the magnetic iron oxide. The electromagnetic wave absorbingsheet has a flexibility evaluation value F (g/mm²) of more than 0 and 6or less. The flexibility evaluation value F (g/mm²) is determined bymeasuring an applied weight (g) that is required to bend a ribbon-likeelectromagnetic wave absorbing sheet in the elastic deformation regionso that a distance d between the inner surfaces of the ribbon-like sheetat a position L spaced 10 mm from the bent portion of the ribbon-likesheet is 10 mm, and dividing the applied weight (g) by thecross-sectional area D (mm²) of the ribbon-like sheet.

With this configuration, the electromagnetic wave absorbing sheet of thepresent disclosure can absorb electromagnetic waves at high frequenciesin and above the millimeter wave band by the magnetic resonance of themagnetic iron oxide. Moreover, due to the use of the magnetic iron oxideand the resin binder, the electromagnetic wave absorbing sheet has aflexibility evaluation value F (g/mm²) of more than 0 and 6 or less, andthus can be significantly bent without causing plastic deformation. Theflexibility evaluation value F (g/mm²) is determined by measuring anapplied weight (g) that is required to bend a ribbon-likeelectromagnetic wave absorbing sheet in the elastic deformation regionso that a distance d between the inner surfaces of the ribbon-like sheetat a position L spaced 10 mm from the bent portion of the ribbon-likesheet is 10 mm, and dividing the applied weight (g) by thecross-sectional area D (mm²) of the ribbon-like sheet. Consequently,even if the electromagnetic wave absorbing sheet is placed on a curvedsurface of, e.g., the inner or outer surface of an equipment case thathouses the electronic circuits to be protected, or if theelectromagnetic wave absorbing sheet is very distorted, e.g., during therearrangement, the electromagnetic wave absorbing layer is not easilycracked or fractured, so that the electromagnetic wave absorbing sheetcan be highly flexible and have resistance to plastic deformation.

In the electromagnetic wave absorbing sheet of the present disclosure,the glass transition temperature (Tg) of the resin binder is preferably0° C. or less. This allows the electromagnetic wave absorbing sheet tohave sufficient flexibility for practical use.

The flexibility evaluation value F is preferably 1.5 or more and 3.5 orless. This makes the electromagnetic wave absorbing sheet highlypracticable because it can achieve both ease of handling (i.e.,self-supporting properties), which helps the user to carry theelectromagnetic wave absorbing sheet, and high adaptability(flexibility) to the shape of the place where the electromagnetic waveabsorbing sheet is to be arranged, while maintaining the electromagneticwave absorption performance at high frequencies in and above themillimeter wave band.

In the electromagnetic wave absorbing sheet of the present disclosure,it is preferable that the content of the magnetic iron oxide in theelectromagnetic wave absorbing layer is 30% by volume or more, and thecontent of all inorganic filler powders, including the magnetic ironoxide, present in the binder in the electromagnetic wave absorbing layeris 50% by volume or less. With this configuration, the electromagneticwave absorbing sheet can have high electromagnetic wave absorptionproperties and high flexibility.

The magnetic iron oxide is preferably an epsilon iron oxide powder. Theepsilon iron oxide has the largest coercive force of all metal oxides,while the natural magnetic resonance frequency is several tens of GHz orhigher. The electromagnetic wave absorbing sheet of the presentdisclosure uses the epsilon iron oxide as an electromagnetic waveabsorbing material that absorbs electromagnetic waves and thus canabsorb electromagnetic waves at high frequencies in and above themillimeter wave band from 30 GHz to 300 GHz.

In this case, the epsilon iron oxide powder is preferably a powder ofthe epsilon iron oxide in which a part of the Fe site is substitutedwith a trivalent metal element. The magnetic resonance frequency of theepsilon iron oxide varies depending on the material that substitutes forthe Fe site. By making use of these properties, the electromagnetic waveabsorbing sheet can absorb electromagnetic waves in the desiredfrequency band.

It is preferable that an adhesive layer is formed on the back side ofthe electromagnetic wave absorbing layer. With this configuration, theelectromagnetic wave absorbing sheet can have high electromagnetic waveabsorption properties and excellent handleability so that it can easilybe placed in a desired location.

In the electromagnetic wave absorbing sheet of the present disclosure,it is preferable that a reflective layer is formed in contact with onesurface of the electromagnetic wave absorbing layer to reflectelectromagnetic waves that have passed through the electromagnetic waveabsorbing layer. This ensures that shielding and absorption ofelectromagnetic waves at frequencies in and above the millimeter waveband can be performed simultaneously. Thus, a so-called reflection-typeelectromagnetic wave absorbing sheet can be provided.

It is further preferable that an adhesive layer is formed on the backside of a laminated body of the electromagnetic wave absorbing layer andthe reflective layer. With this configuration, the reflection-typeelectromagnetic wave absorbing sheet can have high electromagnetic waveabsorption properties and excellent handleability so that it can easilybe placed in a desired location.

Hereinafter, the electromagnetic wave absorbing sheet of the presentdisclosure will be described with reference to the drawings.

Embodiment

[Sheet Configuration]

FIG. 1 is a cross-sectional view illustrating the configuration of anelectromagnetic wave absorbing sheet of an embodiment.

FIG. 1 is illustrated to facilitate the understanding of theconfiguration of the electromagnetic wave absorbing sheet of thisembodiment, and does not necessarily reflect the actual size orthickness of each member in the figure.

The electromagnetic wave absorbing sheet of this embodiment includes anelectromagnetic wave absorbing layer 1 containing a magnetic iron oxide1 a and a resin binder 1 b. As illustrated in FIG. 1, theelectromagnetic wave absorbing sheet also includes an adhesive layer 2.The adhesive layer 2 is formed on the back side (i.e., the lower surfacein FIG. 1) of the electromagnetic wave absorbing layer 1. The adhesivelayer 2 allows the electromagnetic wave absorbing sheet to be attachedto a predetermined location such as the inner or outer surface of a caseof electronic equipment.

The electromagnetic wave absorbing sheet of this embodiment convertselectromagnetic waves into heat energy and dissipates it due to magneticloss that is caused by magnetic resonance of the magnetic iron oxide 1 acontained in the electromagnetic wave absorbing layer 1. Therefore,electromagnetic waves can be absorbed only by the electromagnetic waveabsorbing layer 1. Thus, the electromagnetic wave absorbing sheet may beused as a so-called transmission-type electromagnetic wave absorbingsheet that absorbs electromagnetic waves passing though theelectromagnetic wave absorbing layer 1 without the need for a reflectivelayer on one surface of the electromagnetic wave absorbing layer 1, asillustrated in FIG. 1.

Moreover, the resin binder 1 b constitutes the electromagnetic waveabsorbing layer 1, and the flexibility evaluation value F (g/mm²) of theelectromagnetic wave absorbing sheet in the elastic deformation regionis more than 0 and 6 or less. With this configuration, theelectromagnetic wave absorbing sheet can easily be attached to a curvedsurface or be peeled off and rearranged after it has stuck to thesurface. Thus, the electromagnetic wave absorbing sheet is highlypracticable. The definition and measurement method of the flexibilityevaluation value F will be described in detail later.

Further, the adhesive layer 2 is formed on one surface of theelectromagnetic wave absorbing layer 1. This makes it easier for theelectromagnetic wave absorbing sheet to be attached to a desiredlocation such as the surface of a member that is around the source ofhigh-frequency electromagnetic waves. The presence of the adhesive layer2 is not an essential requirement for the electromagnetic wave absorbingsheet of this embodiment.

[Magnetic Oxide]

In the electromagnetic wave absorbing sheet of this embodiment, theelectromagnetic wave absorbing layer 1 contains the magnetic iron oxide1 a as a member for absorbing electromagnetic waves. For example,epsilon iron oxide or strontium ferrite may be suitable for the magneticiron oxide 1 a that magnetically resonates at frequencies in and abovethe millimeter wave band. In the electromagnetic wave absorbing sheet ofthis embodiment, the magnetic iron oxide 1 a may be preferably aparticulate matter because it is to be dispersed in the resin binder 1b.

The epsilon iron oxide (ε-Fe₂O₃) is a phase that appears between thealpha phase (α-Fe₂O₃) and the gamma phase (γ-Fe₂O₃) in ferric oxide(Fe₂O₃). The epsilon iron oxide is a magnetic material that can beobtained as a single phase by a nanoparticle synthesis method combininga reverse micelle method with a sol-gel method.

The epsilon iron oxide includes fine particles of several nm to severaltens of nm, and still has a coercive force of about 20 kOe at roomtemperature, which is the largest coercive force of all metal oxides.Moreover, the natural magnetic resonance of the epsilon iron oxideoccurs due to a gyromagnetic effect based on precession at frequenciesof several tens of GHz or higher, corresponding to the millimeter waveband. Therefore, the epsilon iron oxide has a great effect of absorbingelectromagnetic waves at high frequencies in and above the millimeterwave band from 30 GHz to 300 GHz.

In the epsilon iron oxide, a part of the Fe site of the crystal issubstituted with a trivalent metal element such as aluminum (Al),gallium (Ga), rhodium (Rh), or indium (In). This substitution can changethe magnetic resonance frequency; i.e., the frequency of electromagneticwaves to be absorbed by the epsilon iron oxide when it is used as anelectromagnetic wave absorbing material.

FIG. 2 shows the relationship between the coercive force Hc and thenatural resonance frequency f of the epsilon iron oxide when the metalelement that substitutes for the Fe site is changed. The naturalresonance frequency f substantially coincides with the frequency ofelectromagnetic waves to be absorbed.

It is evident from FIG. 2 that the natural resonance frequency of theepsilon iron oxide in which a part of the Fe site is substituted with ametal element varies depending on the type of the metal element and thesubstitution amount, and that the coercive force of the epsilon ironoxide increases with an increase in the natural resonance frequency.

More specifically, the gallium-substituted epsilon iron oxide(ε-Ga_(x)Fe_(2-x)O₃) has an absorption peak in a frequency band fromabout 30 GHz to 150 GHz by adjusting the substitution amount “x”. Thealuminum-substituted epsilon iron oxide (ε-Al_(x)Fe_(2-x)O₃) has anabsorption peak in a frequency band from about 100 GHz to 190 GHz byadjusting the substitution amount “x”. Therefore, the frequency ofelectromagnetic waves to be absorbed can be set to a desired value byselecting the type of the element that substitutes for the Fe site ofthe epsilon iron oxide and further adjusting the substitution amount ofFe so that the natural resonance frequency of the epsilon iron oxidewill be the absorption frequency of the electromagnetic wave absorbingsheet. Moreover, the use of the rhodium-substituted epsilon iron oxide(ε-Rh_(x)Fe_(2-x)O₃) can shift the frequency band of electromagneticwaves to be absorbed to 180 GHz or higher.

Epsilon iron oxides, including those in which a part of the Fe site issubstituted with metal, are on the market and easily available.

The oxide magnetic material 1 a contained in the electromagnetic waveabsorbing layer 1 of the electromagnetic wave absorbing sheet of thisembodiment may be strontium ferrite. The strontium ferrite is acomposite oxide of strontium and iron and is commonly used as a magnetmaterial. The strontium ferrite has a hexagonal crystal structure with asize of about several μm. The strontium ferrite magnetically resonateswith electromagnetic waves at frequencies of several tens of GHz and canabsorb the electromagnetic waves.

[Resin Binder]

The resin binder 1 b contained in the electromagnetic wave absorbinglayer 1 may be a resin material such as epoxy resin, polyester resin,polyurethane resin, acrylic resin, phenol resin, melamine resin, orrubber resin.

More specifically, examples of the epoxy resin include a compoundobtained by epoxidation of hydroxyl groups at both ends of bisphenol A.Examples of the polyurethane resin include polyester urethane resin,polyether urethane resin, polycarbonate urethane resin, and epoxyurethane resin. Examples of the acrylic resin include a functional groupcontaining methacrylic polymer obtained by copolymerization of alkylacrylate and/or alkyl methacrylate, both of which are methacrylic resinhaving an alkyl group with 2 to 18 carbon atoms, and a functional groupcontaining monomer, and optionally other modifying monomerscopolymerizable with them.

Examples of the rubber resin used as a binder include the following:styrene-based thermoplastic elastomers such as SIS (styrene-isopreneblock copolymer) and SBS (styrene-butadiene block copolymer); petroleumsynthetic rubber such as EPDM (ethylene-propylene-diene-rubber); andother rubber materials such as acrylic rubber and silicone rubber.

Among these various resins, the polyester resin is preferred because ofits high flexibility. In terms of environmental protection, the resinused as a binder is preferably free of halogen. i.e., a halogen-freeresin. The above resin materials are common materials for binders ofresin sheets, and thus are easily available.

In this embodiment, the flexibility evaluation value F (g/mm²) of theelectromagnetic wave absorbing sheet in the elastic deformation regionis more than 0 and 6 or less. 1 b achieve such flexibility of theelectromagnetic wave absorbing sheet, the glass transition temperature(Tg) of the binder material is 0 degrees (Celsius) or less, and morepreferably −5 degrees or less.

In general, the glass transition temperature of a resin materialcontaining a metal powder or iron oxide is likely to be higher than thatof a resin material alone, which does not contain a metal powder.Therefore, as in the case of the electromagnetic wave absorbing sheet ofthis embodiment, when the electromagnetic wave absorbing layer containsa predetermined amount (e.g., 30% by volume) or more of the magneticiron oxide such as an epsilon iron oxide powder or a strontium ferritepowder present in the resin binder, the glass transition temperature ofthe resin binder should be within the above range in order to ensure theflexibility of the electromagnetic wave absorbing sheet during actualuse. Thus, the electromagnetic wave absorbing sheet can have goodflexibility.

In this embodiment, the glass transition temperature (Tg) is determinedin the following manner. A sample of 2 mm (width)×20 mm (length) isprepared. The thickness of the sample is measured with a micrometer.Then, using Rheogel-E4000 (product name) manufactured by UBM, the sampleis measured with a tensile jig (measurement jig) under the conditionsthat the temperature is in the range of −70° C. to 20° C., the rate oftemperature rise is 3° C./min, and the frequency is 10 Hz.

[Electromagnetic Wave Absorbing Layer]

In the electromagnetic wave absorbing sheet of this embodiment, theelectromagnetic wave absorbing layer 1 contains the magnetic iron oxide1 a as an electromagnetic wave absorbing material. The magnetic ironoxide 1 a includes fine metal oxide particles with a particle size ofseveral nm to several μm such as an epsilon iron oxide powder or astrontium ferrite powder. Thus, it is important to adequately dispersethe magnetic iron oxide 1 a in the binder 1 b during the formation ofthe electromagnetic wave absorbing layer 1.

For this reason, the electromagnetic wave absorbing layer 1 contains aphosphate compound. Examples of the phosphate compound include thefollowing: arylsulfonic acids such as phenylphosphonic acid andphenylphosphonic dichloride; alkylphosphonic acids such asmethylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, andpropylphosphonic acid; and polyfunctional phosphonic acids such ashydroxyethanediphosphonic acid and nitrotris methylenephosphonic acid.Since these phosphate compounds are flame-retardant and function as adispersing agent for the fine magnetic iron oxide 1 a, the magnetic ironoxide 1 a can be adequately dispersed in the binder.

Specifically, the dispersing agent may be, e.g., phenylphosphonic acid(PPA) manufactured by FUJIFILM Wako Pure Chemical Corporation or NissanChemical Corporation, or oxidized phosphoric acid ester “JP-502”(product name) manufactured by JOHOKIU CHEMICAL CO., LTD.

As an example, the electromagnetic wave absorbing layer 1 may becomposed of 2 to 50 parts of the resin binder and 0.1 to 15 parts of thephosphate compound with respect to 100 parts of the epsilon iron oxidepowder. If the content of the resin binder is less than 2 parts, theepsilon iron oxide powder cannot be well dispersed, and the shape of theelectromagnetic wave absorbing sheet cannot be maintained. If thecontent of the resin binder is more than 50 parts, the volume content ofthe epsilon iron oxide powder in the electromagnetic wave absorbingsheet is reduced and magnetic permeability becomes low, so that theelectromagnetic wave absorption effect is reduced.

If the content of the phosphate compound is less than 0.1 parts themagnetic iron oxide cannot be well dispersed in the resin binder. If thecontent of the phosphate compound is more than 15 parts, the effect ofadequately dispersing the magnetic iron oxide becomes saturated. Thevolume content of the magnetic iron oxide in the electromagnetic waveabsorbing sheet is reduced and magnetic permeability becomes low, sothat the electromagnetic wave absorption effect is reduced.

When the content of the resin binder and the content of the phosphatecompound are set within the above ranges, respectively it is possible toimprove the dispersibility of the epsilon iron oxide powder and toreduce the maximum particle size or the average particle size.Consequently the electromagnetic wave absorbing sheet can have higherflexibility

[Method for Producing Electromagnetic Wave Absorbing Layer]

Hereinafter, an example of a method for producing the electromagneticwave absorbing layer 1 of the electromagnetic wave absorbing sheet ofthis embodiment will be described. In the method, a magnetic coatingmaterial containing at least the magnetic iron oxide 1 a and the resinbinder 1 b is prepared. The magnetic coating material is applied in apredetermined thickness, dried, and then calendered to form theelectromagnetic wave absorbing layer 1. The following example uses anepsilon iron oxide powder as the magnetic iron oxide 1 a.

First, the magnetic coating material is prepared.

The epsilon iron oxide powder, the phosphate compound (dispersingagent), and the resin binder are mixed and kneaded. The resultantkneaded material is diluted, further dispersed, and then filteredthrough a filter, thus providing the magnetic coating material. Thekneaded material may be obtained, e.g., by kneading the above mixturewith a pressure batch kneader. The kneaded material may be dispersed,e.g., with a sand mill that is filled with beads such as zirconia toform a dispersion liquid. In this case, a crosslinking agent may beadded as needed.

The magnetic coating material thus prepared is applied to a supporthaving releasability by using a table coater, a bar coater, or the like.The support may be, e.g., a polyethylene terephthalate (PET) sheet thathas a thickness of 38 μm and is treated with a silicone coating.

Then, the magnetic coating material in the wet state is dried at 80° C.and further calendered at a predetermined temperature with a calendar.Thus, an electromagnetic wave absorbing layer is formed on the support.

For example, the thickness of the magnetic coating material may be 1 mmwhen it is applied in a wet state to the support. In this case, themagnetic coating material has a thickness of 400 μm after drying, andthe electromagnetic wave absorbing layer has a thickness of 300 μm aftercalendering.

In this manner, the electromagnetic wave absorbing layer 1 can be formedin which the nano-order fine epsilon iron oxide (the magnetic oxide 1 a)is adequately dispersed in the resin binder 1 b.

There is another method for preparing the magnetic coating material. Thecomponents of a magnetic coating material including at least themagnetic iron oxide, the phosphate compound (dispersing agent), and theresin binder may be mixed at a high speed with a high-speed stirrer. Theresultant mixture may be subjected to a dispersion treatment with a sandmill, thus providing the magnetic coating material.

[Adhesive Layer]

As illustrated in FIG. 1, the electromagnetic wave absorbing sheet ofthis embodiment includes the adhesive layer 2 that is formed on the backside of the electromagnetic wave absorbing layer 1.

The adhesive layer 2 allows the electromagnetic wave absorbing layer 1to be attached to a desired position of, e.g., the inner surface of acase that houses electric circuits or the inner or outer surface ofelectrical equipment. In particular, since the electromagnetic waveabsorbing layer 1 has flexibility, the electromagnetic wave absorbingsheet of this embodiment can also easily be attached to a curved surfacedue to the presence of the adhesive layer 2. This can improve the easeof handling of the electromagnetic wave absorbing sheet.

The adhesive layer 2 may be made of any known materials used foradhesive layers of adhesive tapes or the like, including. e.g., anacrylic adhesive, a rubber adhesive, and a silicone adhesive. Moreover,a tackifier or a crosslinking agent may also be used to adjust theadhesion force to the adherend or to reduce the adhesive residue. Theadhesion force to the adherend is preferably 5 N/10 mm to 12 N/10 mm. Ifthe adhesion force is less than 5 N/10 mm, the electromagnetic waveabsorbing sheet is likely to be peeled off or displaced from theadherend. If the adhesion force is more than 12 N/10 mm, theelectromagnetic wave absorbing sheet is not likely to be peeled off theadherend.

The thickness of the adhesive layer 2 is preferably 20 μm to 100 μm. Ifthe thickness of the adhesive layer is less than 20 μm, the adhesionforce is reduced, and thus the electromagnetic wave absorbing sheet islikely to be peeled off or displaced from the adherend. If the thicknessof the adhesive layer is more than 100 μm, the flexibility of the wholeelectromagnetic wave absorbing sheet may be reduced. Moreover, if thethickness of the adhesive layer 2 is large, the electromagnetic waveabsorbing sheet is not likely to be peeled off the adherend. Further, ifthe cohesion of the adhesive layer 2 is small, an adhesive residue maybe left on the adherend after the electromagnetic wave absorbing sheetis removed.

In the present specification, the adhesive layer 2 may be removably orunremovably attached to the adherend.

The electromagnetic wave absorbing sheet may be attached to apredetermined surface even without the adhesive layer 2. For example, byimparting adhesion to the surface of a member to which theelectromagnetic wave absorbing sheet adheres or by using a double-sidedadhesive tape or an adhesive, the electromagnetic wave absorbing sheetcan be attached to a predetermined location. In this regard, it is clearthat the adhesive layer 2 is not an essential component of theelectromagnetic wave absorbing sheet of this embodiment.

[Flexibility of Electromagnetic Wave Absorbing Sheet]

Next, the flexibility of the electromagnetic wave absorbing sheet ofthis embodiment will be described. In the following description, anelectromagnetic wave absorbing sheet that does not include the adhesivelayer 2, but only includes the electromagnetic wave absorbing layer 1 isused as a measuring object.

FIG. 3 is a diagram for explaining the measurement of a flexibilityevaluation value F that indicates the degree of flexibility of theelectromagnetic wave absorbing sheet of this embodiment.

The electromagnetic wave absorbing sheet in the form of a ribbon with alength of 100 mm and a width of 20 mm is used for the measurement. Asillustrated in FIG. 3, the ribbon-like electromagnetic wave absorbingsheet is folded lengthwise in the middle so that both ends in thelongitudinal direction of the sheet overlap each other. Then, anexternal force required to maintain this state is determined. Further,the external force is divided by the cross-sectional area of theelectromagnetic wave absorbing sheet to give the flexibility evaluationvalue F of the electromagnetic wave absorbing sheet.

For example, as illustrated in FIG. 3, the electromagnetic waveabsorbing sheet to be measured is placed on a measurement stand 11 of anelectronic balance, and a self weight of the electromagnetic waveabsorbing sheet is measured while no external force is applied. Next, aweight added to the electronic balance is measured when an externalforce is applied to deform the electromagnetic wave absorbing sheet. Theself weight of the electromagnetic wave absorbing sheet is subtractedfrom the measurement result to find the applied weight that is requiredto maintain the folded state of the electromagnetic wave absorbingsheet.

A plate member 12 is located on the upper side of the electromagneticwave absorbing sheet so that the electromagnetic wave absorbing sheetremains in the predetermined folded state, as illustrated in FIG. 3. Anexternal force is applied vertically downward to the plate member 12, asindicated by the white arrow 13 in FIG. 3. Then, the magnitude of theexternal force 13 is measured as a weight when a distance d between theinner surfaces of the folded sheet at a position spaced L (10 mm) fromthe outer edge of the bent portion of the electromagnetic wave absorbingsheet 1 is 10 mm. The measured weight is divided by the cross-sectionalarea D (mm²) of the electromagnetic wave absorbing sheet to give theflexibility evaluation value F (g/mm²). The flexibility evaluation valueF (g/mm²) is determined at a temperature of 23° C. and a humidity of 50%Rh.

For example, the electronic balance reads 6 gw (gram-weight) when theexternal force 13 is applied until the electromagnetic wave absorbingsheet with a thickness of 100 μm (=0.1 mm) is brought into the state asillustrated in FIG. 3. In this case, the cross-sectional area D of theelectromagnetic wave absorbing sheet is calculated by 20 (mm)×0.1 (mm)=2(mm²). Therefore, the flexibility evaluation value F is determined by6÷2=3 (g/mm²). If this flexibility evaluation value F is more than 0 and6 or less, the electromagnetic wave absorbing sheet can be considered tohave good flexibility. If the value F is more than 0 and 4 or less, theelectromagnetic wave absorbing sheet is preferred because it can achieveboth self-supporting properties and flexibility. If the value F is 1.5or more and 3.5 or less, the electromagnetic wave absorbing sheet canhave better flexibility.

The reason that the flexibility evaluation value F is more than 0 is asfollows. If the value F is 0, the electromagnetic wave absorbing sheetis bent under its own weight, and cannot form a shape such that twoparts extending from the bent portion to their respective ends are keptin parallel with each other, as illustrated in FIG. 3. Thiselectromagnetic wave absorbing sheet becomes too soft to support its ownweight. Accordingly, it is difficult to handle the electromagnetic waveabsorbing sheet when the user carries it or attach it to a predeterminedlocation. If the value F is more than 6, a large force is required tobend the electromagnetic wave absorbing sheet, which leads to poorworkability.

Regarding the lower limit of the flexibility evaluation value F, thecalculated value F (e.g., 0.1 or 0.01) will be “0” if it is rounded toone significant figure. However, this case should be distinguished fromthe case where the measured value itself is “0”. As described above, theflexibility evaluation value is defined as F=0 when the electromagneticwave absorbing sheet is bent under its own weight. If the value F ismore than 0 (no matter how small it is, such as 0.01), a force should berequired to press the electromagnetic wave absorbing sheet. Thus, in thepresent invention, the flexibility evaluation value F larger than 0means that some external force is required to bend the electromagneticwave absorbing sheet so that the distance d between the inner surfacesof the folded sheet at a position spaced L (10 mm) from the outer edgeof the bent portion is 10 mm.

The measurement of the flexibility evaluation value F in FIG. 3 isperformed based on the premise that the electromagnetic wave absorbingsheet to be measured is in the elastic deformation region. In otherwords, it is important that the electromagnetic wave absorbing sheetreturns to its initial shape once the plate member 12 is removed aftermeasuring the flexibility evaluation value F. If the electromagneticwave absorbing sheet is plastically deformed under the applied externalforce and cannot be restored to its initial shape after the removal ofthe plate member 12, or if there are apparent defects such as cracks inthe outer surface of the bent portion of the sheet, the electromagneticwave absorbing sheet is regarded as not having the predeterminedflexibility evaluation value.

In the initial state, when the electromagnetic wave absorbing sheet isplaced on the measurement stand of the electronic balance and bent sothat one end lies on top of the other, the sheet can be temporarilyfolded in half with both ends overlapping each other. However, if theradius of the bent portion is large, the distance d of theelectromagnetic wave absorbing sheet at a position spaced L (10 mm) fromthe outer edge of the bent portion may be larger than 10 mm. Thisrepresents the initial shape of the electromagnetic wave absorbing sheetwith relatively high flexibility. For lower flexibility, even if anattempt is made to align both ends of the electromagnetic wave absorbingsheet, they do not overlap, but are spread apart. Consequently, thedistance between the inner surfaces of the sheet is greater at the endportion than at the bent portion. For much lower flexibility theelectromagnetic wave absorbing sheet may return to its linear formimmediately upon removal of the force that has been applied to the sheetto put one end on top of the other. Thus, in the present specification,“the electromagnetic wave absorbing sheet in the elastic deformationregion” means that each electromagnetic wave absorbing sheet is in astate where it can return to its initial shape in accordance with thedegree of flexibility after the external force has been removed.

Even if the electromagnetic wave absorbing sheet includes an adhesivelayer and a reflective layer (as will be described later), these layerscan be extremely thinner than the electromagnetic wave absorbing layer,and therefore the formation of the adhesive layer or reflective layerhas a small effect on the flexibility evaluation value F.

As described above, only a slight external force is required to bend theelectromagnetic wave absorbing sheet of this embodiment. At the sametime, it is also important for the electromagnetic wave absorbing sheetto have high restorability so that the sheet that has been verydistorted can return to its original shape when the external force isreleased. To achieve such high restorability, it is preferable that theaverage particle size of the magnetic iron oxide contained in theelectromagnetic wave absorbing layer is 5 to 50 nm for epsilon ironoxide and 1 to 5 μm for strontium ferrite.

FIG. 4 is a conceptual diagram for explaining the effect of the particlesize of the magnetic iron oxide contained in the electromagnetic waveabsorbing layer on the flexibility of the electromagnetic wave absorbingsheet.

FIG. 4(a) illustrates a state where the particle size of the magneticiron oxide is sufficiently small FIG. 4(b) illustrates a state where theparticle size of the magnetic iron oxide is large. FIGS. 4(a) and 4(b)illustrate the shape of the electromagnetic wave absorbing sheet that isbent and pressed between two plate members to reduce the distancebetween the inner surfaces of the sheet in order to determine theflexibility evaluation value F, as illustrated in FIG. 3.

As illustrated in FIG. 4(a), when the average particle size of themagnetic iron oxide 1 a contained in the resin binder 1 b issufficiently small, the electromagnetic wave absorbing sheet (i.e., theelectromagnetic wave absorbing layer 1) is smoothly curved so that twoparts extending from the substantially semicircular bent portion totheir respective ends form parallel lines.

On the other hand, as illustrated in FIG. 4(b), when the particle sizeof a magnetic iron oxide 1 a′ contained in a resin binder 1 b′ is large,particles of the magnetic iron oxide come into contact with each otherin the bent portion of the electromagnetic wave absorbing sheet. Thus,since the radius of the bent portion is not fully reduced, two partsextending from the bent portion to their respective ends do not formparallel lines and are spread apart at the end portion. In this state,when the force (i.e., the external force 13 in FIG. 3) is increased sothat the electromagnetic wave absorbing sheet is pressed between theplate members to reduce the distance between the two ends of the sheet,the resin binder causes cracks or breakage in the bent portion,resulting in plastic deformation of the electromagnetic wave absorbingsheet (i.e., the electromagnetic wave absorbing layer 1).

FIGS. 4(a) and 4(b) are merely representations of the states of themagnetic iron oxide in the bent portion, and the ratio of the particlesize of the magnetic iron oxide to the thickness of the electromagneticwave absorbing layer differs from the actual measured value.

EXAMPLES

Next, electromagnetic wave absorbing sheets of this embodiment wereactually produced and their flexibility evaluation values were measured.The results of the measurement will be described below.

The following three electromagnetic wave absorbing sheets of thisembodiment were produced: an electromagnetic wave absorbing sheet inwhich epsilon iron oxide was used as a magnetic oxide and polyester wasused as binder (Example 1); an electromagnetic wave absorbing sheet inwhich polyurethane was used as a binder (Example 2); and anelectromagnetic wave absorbing sheet in which strontium ferrite was usedas an magnetic oxide and silicone rubber was used as a binder (Example3). In Examples 1, 2 and 3, the electromagnetic wave absorbing sheetsdid not include an adhesive layer, but only included an electromagneticwave absorbing layer. The amounts of components in the composition ofeach electromagnetic wave absorbing sheet are as follows.

(Example 1) Magnetic iron oxide 40.0 g Epsilon iron oxide powder 73.1 gBinder Polyester VYLON 55SS (product name) manufactured by TOYOBO CO.,LTD. Tg: −15° C., solid content: 25.6 g, solvent : 47.5 g Phenylsulfonicacid 2.0 g (PPA: dispersing agent) Methyl ethyl ketone 20.1 g (MEK:solvent) (Example 2) Magnetic iron oxide 40.0 g Epsilon iron oxidepowder 50.6 g Binder Polyurethane VYLON UR 8700 (product name)manufactured by TOYOBO CO., LTD. Tg: −22° C., solid content: 15.2 g,solvent: 85.4 g Phenylsulfonic acid 2.0 g (PPA: dispersing agent) Methylethyl ketone 2.7 g (MEK: solvent) (Example 3) Magnetic iron oxide 100.0g Strontium ferrite powder 30.0 g Binder Silicone rubber KE-510U(product name) manufactured by Shin-Etsu Chemical Co., Ltd. Tg: −125° C.Curing agent 0.9 g C-8A (product name) manufactured by Shin-EtsuChemical Co., Ltd.

The flexibility evaluation values F of the three electromagnetic waveabsorbing sheets were measured by the method as illustrated in FIG. 3.

The electromagnetic wave absorbing sheet of Example 1, using polyesteras the binder, had a flexibility evaluation value F of 1.4 (g/mm²). Theelectromagnetic wave absorbing sheet of Example 2, using polyurethane asthe binder, had a flexibility evaluation value F of 2.7 (g/mm²). Theelectromagnetic wave absorbing sheet of Example 3, using silicone rubberas the binder, had a flexibility evaluation value F of 1.1 (g/mm²). Theresults confirmed that all the flexibility evaluation values F fell inthe range (more than 0 and 6 or less) of the flexibility evaluationvalue of the electromagnetic wave absorbing sheet of this embodiment.

The electromagnetic wave absorbing sheets of Examples 1 and 2 had goodflexibility and were also able to absorb, e.g., electromagnetic waves atfrequencies in the millimeter wave band of 75 GHz due to the use of theepsilon iron oxide as an electromagnetic wave absorbing material.Moreover, the electromagnetic wave absorbing sheet of Example 3 was alsoable to absorb electromagnetic waves at frequencies in the millimeterwave band of 76 GHz due to the use of the strontium ferrite as anelectromagnetic wave absorbing material.

As comparative examples, the flexibility evaluation values ofelectromagnetic wave absorbing sheets were measured. Similarly to theelectromagnetic wave absorbing sheets of Examples 1, 2, and 3, theelectromagnetic wave absorbing sheets used in the comparative examplescontained the epsilon iron oxide (magnetic iron oxide) and were able toabsorb electromagnetic waves at high frequencies in and around the 75GHz band. Each of the electromagnetic wave absorbing sheets of thecomparative examples was a so-called electromagnetic wave interferencetype (λ/4 type) electromagnetic wave absorbing sheet, which absorbselectromagnetic waves by shifting the phase of reflected waves by ½wavelength so that the reflected waves and incident waves on theelectromagnetic wave absorbing sheet cancel each other out. In the firstcomparative example, a rubber-type electromagnetic wave absorbing sheet(“SA76” (product name) manufactured by FDK CORPORATION) was used and theflexibility evaluation value F was measured after the metal layer(reflective layer) was removed. As a result, the flexibility evaluationvalue F of this electromagnetic wave absorbing sheet was 8.9 (g/mm²). Inthe second comparative example, an electromagnetic wave absorbing sheet(“EC-SORB SF-76.5 MB” (product name) manufactured by E & C EngineeringK.K) was used and the flexibility evaluation value F was measured afterthe metal layer (reflective layer) was removed. As a result, theflexibility evaluation value F of this electromagnetic wave absorbingsheet was 7.1 (g/mm²). Thus, both the existing electromagnetic waveabsorbing sheets used in the comparative examples had lower flexibilitythan the electromagnetic wave absorbing sheets of Examples 1, 2, and 3.

To determine a preferred range of the flexibility evaluation value F, anelectromagnetic wave absorbing sheet of Example 4 and an electromagneticwave absorbing sheet of the third comparative example (ComparativeExample 3) were produced by using an epsilon iron oxide powder as amagnetic iron oxide. The amounts of components in the composition ofeach electromagnetic wave absorbing sheet are as follows. Theseelectromagnetic wave absorbing sheets were produced in the same manneras the electromagnetic wave absorbing sheets of Examples 1, 2, and 3.

(Example 4) Magnetic iron oxide 40.0 g Epsilon iron oxide powder 73.1 gBinder Polyester VYLON 50SS (product name) manufactured by TOYOBO CO.,LTD. Tg: −3° C., solid content: 15.2 g, solvent: 35.4 g Phenylsulfonicacid 2.0 g (PPA: dispersing agent) Methyl ethyl 20.1 g ketone (MEK:solvent) (Comparative Example 3) Magnetic iron oxide 40.0 g Epsilon ironoxide powder 50.6 g Binder Polyurethane VYLON UR 3200 (product name)manufactured by TOYOBO CO., LTD. Tg: 4° C., solid. content: 15.2 g,solvent: 35.4 g Phenylsulfonic acid 2.0 g (PPA: dispersing agent)

The electromagnetic wave absorbing sheet of Example 4 had a flexibilityevaluation value F of 3.8 (g/mm²). This electromagnetic wave absorbingsheet returned to its initial shape upon removal of the external forceafter finishing the measurement of the flexibility evaluation value F.On the other hand, the electromagnetic wave absorbing sheet ofComparative Example 3 had a flexibility evaluation value F of 6.3(g/mm²). When an external force was applied to this electromagnetic waveabsorbing sheet, the radius of the bent portion was not readily reduced.After the flexibility evaluation value F was measured as the distance dof the electromagnetic wave absorbing sheet at a position spaced L (10mm) from the outer edge of the bent portion was 10 mm, cracks wereobserved in the surface of the electromagnetic wave absorbing layer 1 onthe outside of the bent portion.

The above results confirmed that the electromagnetic wave absorbingsheets with a flexibility evaluation value F of 1 or more and 4 or lesshad both sufficient flexibility and self-supporting properties. If theflexibility evaluation value F was more than 6, the repulsion of theelectromagnetic wave absorbing sheet was too large. Therefore, theelectromagnetic wave absorbing layer might be damaged when theelectromagnetic wave absorbing sheet was very distorted, e.g., duringthe rearrangement of the sheet.

In the electromagnetic wave absorbing sheets of Examples 1, 2 and 4 andComparative Example 3, the blending ratio of the epsilon iron oxidepowder to the binder material was determined so that the content of theepsilon iron oxide in the finished electromagnetic wave absorbing layerwas about 40% by volume. Moreover, in the electromagnetic wave absorbingsheet of Example 3, the blending ratio of the strontium iron oxidepowder to the binder material was determined so that the content of thestrontium ferrite in the finished electromagnetic wave absorbing layerwas about 40% by volume.

The electromagnetic wave absorbing sheet of this embodiment absorbsincident electromagnetic waves by magnetic resonance of the magneticiron oxide contained in the electromagnetic wave absorbing layer. Thus,the electromagnetic wave absorption properties are reduced withdecreasing the content of the magnetic iron oxide in the electromagneticwave absorbing layer. If the intensity of the transmitted waves throughthe electromagnetic wave absorbing sheet is reduced by 15 dB relative tothe intensity of the incident waves, the electromagnetic wave absorbingsheet absorbs 90% of electromagnetic waves. To achieve theelectromagnetic wave absorbing sheet having such electromagnetic waveabsorption properties, the content of the magnetic iron oxide in theelectromagnetic wave absorbing layer should be at least 40% by volume,which can be used as a reference. When the content of the magnetic ironoxide is 40% by volume or more, the value of the imaginary part (μ″) ofmagnetic permeability of the electromagnetic wave absorbing layer isincreased, and thus the electromagnetic wave absorbing sheet can havehigh electromagnetic wave absorption properties.

On the other hand, the flexibility of the electromagnetic wave absorbingsheet is reduced with increasing the filler powder components containedin the electromagnetic wave absorbing layer. The filler powdercomponents include. e.g., the magnetic iron oxide and an inorganicfiller present in the binder material. Studies conducted by the presentinventors showed that if the content of all inorganic filler powders ismore than 50% by volume, the flexibility may not be sufficient for theelectromagnetic wave absorbing sheet. Needless to say, the degree offlexibility of the electromagnetic wave absorbing layer varies dependingon the size or shape of the inorganic filler powders. Moreover, as thecontent of the magnetic iron oxide increases, the electromagnetic waveabsorption properties are improved. Thus, it is preferable that theelectromagnetic wave absorbing layer is formed so as not to include asolid content such as a filler other than the magnetic iron oxide.

[Modified Example of Electromagnetic Wave Absorbing Sheet]

Hereinafter, a modified example of the electromagnetic wave absorbingsheet of this embodiment will be described.

The electromagnetic wave absorbing sheet of the present disclosureabsorbs electromagnetic waves by magnetic resonance of the magnetic ironoxide. The magnetic iron oxide is used as an electromagnetic waveabsorbing material and constitutes, along with the resin binder, theelectromagnetic wave absorbing layer. Thus, the electromagnetic waveabsorbing sheet may include a reflective layer (e.g., a metal layer)that reflects electromagnetic waves. The reflective layer may beprovided on the surface of the electromagnetic wave absorbing layer thatis on the opposite side to the surface on which electromagnetic wavesare incident.

FIG. 5 is a cross-sectional view illustrating a modified example of theconfiguration of the electromagnetic wave absorbing layer of thisembodiment.

Like FIG. 1 illustrating the configuration of the electromagnetic waveabsorbing sheet of this embodiment, FIG. 5 is illustrated to facilitatethe understanding of the configuration of the electromagnetic waveabsorbing sheet, and does not necessarily reflect the actual size orthickness of each member in the figure. The members corresponding tothose illustrated in FIG. 1 are denoted by the same reference numerals,and the detailed explanation will not be repeated.

The electromagnetic wave absorbing sheet of the modified exampleincludes the electromagnetic wave absorbing layer 1 containing themagnetic iron oxide 1 a and the resin binder 1 b. The magnetic ironoxide 1 a magnetically resonates at frequencies in and above themillimeter wave band. The electromagnetic wave absorbing sheet alsoincludes a reflective layer 3. The reflective layer 3 is formed on theback side (i.e., the lower surface in FIG. 5) of the electromagneticwave absorbing layer 1 and is in contact with the surface of theelectromagnetic wave absorbing layer 1.

As illustrated in FIG. 5, the electromagnetic wave absorbing sheet ofthe modified example further includes the adhesive layer 2. The adhesivelayer 2 is formed on the back side of the reflective layer 3. Theadhesive layer 2 allows the electromagnetic wave absorbing sheet to beattached to a predetermined location.

The reflective layer 3 may be a metal layer that is formed in closecontact with the back side of the electromagnetic wave absorbing layer1. However, since the flexibility evaluation value F of theelectromagnetic wave absorbing sheet in the elastic deformation regionis more than 0 and 6 or less, it is difficult to use a metal plate asthe reflective layer 3. Therefore, the reflective layer 3 may beprovided as any of the following: a metal foil that is disposed in closecontact with the back side of the electromagnetic wave absorbing layer1; a metal deposited film that is deposited on the back side of theelectromagnetic wave absorbing layer 1; and a metal deposited film thatis formed on the surface of a non-metal sheet member (e.g., a resin)that faces the electromagnetic wave absorbing layer 1, the non-metalsheet member being located on the back side of the electromagnetic waveabsorbing layer 1.

The type of metal of the reflective layer 3 is not particularly limited,and various metal materials may be used, including the metal materialsgenerally used for electronic components or the like such as aluminum,copper, and chromium. It is more preferable that the reflective layer 3is made of metal with the lowest possible electrical resistance and highcorrosion resistance.

In the electromagnetic wave absorbing sheet of the modified example ofFIG. 5, the formation of the reflective layer 3 on the back side of theelectromagnetic wave absorbing layer 1 can reliably preventelectromagnetic waves from passing through the electromagnetic waveabsorbing sheet. Therefore, the electromagnetic wave absorbing sheet ofthe modified example is particularly suitable for the prevention ofleakage electromagnetic waves emitted from, e.g., electric circuitcomponents driven at a high frequency to the outside.

As described above, the electromagnetic wave absorbing sheet of thisembodiment can adequately absorb electromagnetic waves at highfrequencies in and above the millimeter wave band by magnetic resonanceof the magnetic iron oxide that is used as an electromagnetic waveabsorbing material and contained in the electromagnetic wave absorbinglayer. Moreover, since the flexibility evaluation value F of theelectromagnetic wave absorbing sheet in the elastic deformation regionis more than 0 (g/mm²) and 6 (g/mm²) or less, the electromagnetic waveabsorbing sheet has flexibility high enough for practical use. Thus, theelectromagnetic wave absorbing sheet can easily be attached to a curvedsurface and does not cause breakage or plastic deformation even if it isvery distorted, e.g., during the attachment or rearrangement of thesheet.

In the above description of this embodiment, the magnetic coatingmaterial is prepared, applied and then dried to form the electromagneticwave absorbing layer. The production method of the electromagnetic waveabsorbing layer is not limited to this method for applying the magneticcoating material, and various forming processes such as extrusion may beused.

More specifically, a magnetic iron oxide powder and a binder, andoptionally a dispersing agent, are blended in advance. The blend issupplied through a resin feed opening of an extruder into a plasticcylinder. The extruder may be a common extruder that includes a plasticcylinder, a die provided at the end of the plastic cylinder, a screwrotatably provided in the plastic cylinder, and a drive mechanism fordriving the screw. Then, the blend is plasticized by a band heater ofthe extruder, and the molten material is fed forward by the rotation ofthe screw and forced through the die to produce a sheet-like material.The extruded material is dried, formed under pressure, calendered, etc.,thus providing the electromagnetic wave absorbing layer with apredetermined thickness.

In the electromagnetic wave absorbing sheet of this embodiment, theelectromagnetic wave absorbing layer is a single layer. However, theelectromagnetic wave absorbing layer may be a laminated body of aplurality of layers. This configuration is particularly useful for areflection-type electromagnetic wave absorbing sheet including areflective layer, since the input impedance of the electromagnetic waveabsorbing layer can be controlled to a desired value which is matched tothe impedance of the air. Thus, the electromagnetic wave absorptionproperties of the electromagnetic wave absorbing sheet can be furtherimproved.

INDUSTRIAL APPLICABILITY

The electromagnetic wave absorbing sheet of the present disclosure isuseful as an electromagnetic wave absorbing sheet that absorbselectromagnetic waves at high frequencies in and above the millimeterwave band and has high flexibility.

DESCRIPTION OF REFERENCE NUMERALS

-   -   1 Electromagnetic wave absorbing layer    -   1 a Magnetic iron oxide    -   1 b Binder    -   2 Adhesive layer

1. An electromagnetic wave absorbing sheet comprising: anelectromagnetic wave absorbing layer containing a magnetic iron oxidethat magnetically resonates at frequencies in and above a millimeterwave band and a resin binder, wherein a glass transition temperature(Tg) of the resin binder is 0° C. or less, a content of the magneticiron oxide in the electromagnetic wave absorbing layer is 30% by volumeor more, and a content of all inorganic filler powders, including themagnetic iron oxide, present in the binder in the electromagnetic waveabsorbing layer is 50% by volume or less.
 2. The electromagnetic waveabsorbing sheet according to claim 1, wherein the magnetic iron oxide isan epsilon iron oxide powder and/or a strontium ferrite powder.
 3. Theelectromagnetic wave absorbing sheet according to claim 1 or 2, whereinan adhesive layer is formed on a back side of the electromagnetic waveabsorbing layer.